1. Field of the Invention
The present invention generally concerns tools and methods for accomplishing well treatment such as formation fracturing, proppant slurry injection, gravel packing, etc. More particularly, the present invention concerns a method and apparatus for treating multiple subterranean formations in a single trip, utilizing a coiled or jointed tubing conveyed well treatment tool which provides a means for a circulating or squeeze type treatment, and clean up via reversing out excess slurry by reverse flow or by discharging, i.e., dumping excess slurry internally into the well casing or “rat hole” below the tool. The present invention further concerns well treatment tools that are downhole convertible, thus having the capability of accomplishing selectable landing or locating the tool at a desired service tool position for well treatment and for opening and closing a sliding sleeve valve of the tool for efficient injection fluid flow control.
2. Description of Prior Art
The terms “service tool” or “service tool assembly” as used herein are intended to encompass many differing types of well treatment tools, including tools that are used during subterranean formation fracturing operations, including injection of proppant, i.e., sand/liquid slurry into formation fractures, gravel packing tools, tools for fluid injection operations for chemical treatment of subterranean oil and gas production formations and the like. For purposes of simplicity, to facilitate ready understanding of the present invention, the fracturing tool and the method of treating a subterranean formation are described herein particularly in relation to formation fracturing with proppant slurry.
The term “PBR” as used herein is intended to mean a “polished bore receptacle”, which is an internal cylindrical surface having a close tolerance diameter and having a smooth, i.e., polished surface finish. PBRs are typically employed as internal sealing surfaces which are engaged by annular seal members to establish efficient sealing between components. The term “reverse out” as used herein is intended to mean reversing or changing the direction of flow down the well service tubing to and into the formation for treatment, so that the flow of treatment fluid, including clean-out fluid is diverted upwardly to the surface through a return passage such as may be defined by dual concentric tubing strings.
The term “reversing out”, as employed herein, is intended to mean a fluid injection for well treatment having the capability for accomplishing fluid circulation type treatment where excess treatment fluid is returned to the surface for disposal by a return flow passage, such as can be provided when dual concentric fluid treatment tubing is utilized. In this case the central tubing string is employed to conduct treatment fluid to a downhole treatment tool and the annulus between the central tubing string and an outer tubing string defines the return flow passage.
Typical gravel pack tools are generally equipped with either a load indicating type collet or with a fixed shoulder for locating positions. The load indicating collet is designed to interfere with a fixed profile to the extent that a surface indication of a change of tubing load or resistance may be noted. This type of collet indicator is generally limited to a maximum load (under 20,000 lb) that may not be adequate to compensate for common tubing move effects. A fixed shoulder locator will only provide positioning of a well service tool at one location within a packer and packer extension within a well casing and may prevent repositioning of the well service tool to any point below that location.
An alternative to a selectable landing collar is a selectable collet which is part of a work string rather than part of the permanent completion. Selectable, or downhole convertible collets are currently available but are generally less robust, more susceptible to damage from excessive loads, and may not be suitable for multiple operation applications such as the actuation of other mechanisms like sliding sleeves.
Typical gravel pack jobs are performed with a service tool that seals in two PBRs. One PBR is located below the sliding sleeve valve and the bore of the gravel pack packer and the second PBR is located immediately above the sliding sleeve valve. The gravel pack service tool uses bonded seals rigidly attached to the service tool along its length to provide a seal in the PBRs. A cross-over port is located between two of the service tool seals and forces the gravel pack fluid thru the ports in the gravel pack sleeve. During high flow rate gravel packing jobs erosion damage can occur on the sliding sleeve housing bore which prevents the valve from sealing properly. Damage can also occur to the PBRs where they are exposed to the abrasive gravel packing fluid. In addition, sand can get into the sliding sleeve locking mechanism and prevent it from sliding into place or latching into the closed position.
The invention set forth in U.S. patent application Ser. No. 10/078,963, now issued as U.S. Pat. No. 6,776,239, is a multi-zone service/completion tool assembly, suitable for use in association with the apparatus and method of the present invention.
Components of the multi-zone service/completion tool assembly include:
The service/completion tool assembly uses cup type sealing elements to direct pumped fluids straight into the ported housing. Due to its configuration using less debris sensitive cup type sealing elements, and the ability to reverse out in the treatment position, screen-outs that may occur in the service/completion tool assembly become less critical and do not inhibit tool movement or the operation in subsequent zones. These are key features of the fracturing tool that is being used according to the method of the present invention.
The service tool system incorporates a mechanical valve which is primarily used to selectively close the wash pipe and open ports to enhance reversing out. The mechanical valve is actuated via axial movements of the tubing string. Reversing may be initiated in the treating position without string movement and may be continued while the closing sleeve is shifted closed via tubing string movement. The valve may be configured for either circulating or squeeze type treatments.
The service tool system includes an optional hydraulically actuated dump valve mechanism that allows excess formation treatment slurry to be discarded internally into the well sump or rat hole without any need to reverse the fluid flow in order to remove excess treatment fluid slurry from the service tool. This dump valve feature reduces the potential of sticking the service tool within the well casing, especially in the case of highly deviated or horizontal wellbores, and reduces multiple zone treatment cycle time. The hydraulic dump valve closes and opens a predetermined rate or pressure without the need to manipulate the tubing string to which the service tool is connected for fluid supply and for conveyance within the well casing.
The service tool is positioned precisely in front of the ported housing to construct an ideal flow path for slurry to the reservoir. In order to positively locate the position of the service tool within the service/completion liner, a landing collet must be connected to the service tool. This landing collet serves two purposes: 1) to locate and maintain the ideal position of the service tool for well formation treatment and 2) to close the sleeve valve of the service/completion liner after each zone is treated.
The landing collet is a “set down” collet that selectively locates in a selectable profile collar to maintain a downward set position. A lower zone isolation tool aids to isolate zones below from injection fluids and pressure and also creates a pressure balanced condition which eliminates upward hydraulic loads. The set down feature combined with the pressure balancing lower isolation tool, negates the need for an anchoring device, such as slips or an expandable cone to fix the tool within the well casing and prevent its movement during formation treatment.
The selectable profile (Go/No-go) collars which provide selectable landing shoulders allow the service tool to be precisely placed and maintained in any zone while injecting treatment slurry into a selected formation zone. The service tool may be retrieved or pulled to above the upper packer and returned to any zone set down position.
A one-time landing collet disabling sleeve provides a cost effective means to permit re-entering of lower zones.                In its simplest form the service tool is configured to perform the formation treatment in a “squeeze” mode; i.e. squeeze, meaning all fluids are injected into the formation and not circulated back to surface. A squeeze technique is applicable when a short length of formation interval is being treated. If required, shunting tools such as the AllPAC™/AllFRAC™ tubes can be run on the screen to avoid treatment slurry solids bridges. The addition of a downhole pack off (stripper) and a concentric string between the service tool and the top packer enhances reversing out and permits a circulating type treatment.        
For zones intervals of equal length, (screens plus blank sections), a wash pipe and lower isolation assembly is placed through the screen to isolate the formation below and to cancel hydraulic effects that would otherwise tend to move the tool.                For unequally spaced zones or as an alternative to using wash pipe and the lower isolation assembly, a blind or internally closed screen may be used. With the zone isolated at the screen there is no need for the lower isolation assembly or wash pipe, the service tool ends at the landing collet and will permit only a squeeze type treatment technique to be employed.        
The sleeve seal area of the ported housing is critical; the sleeve must be closed just after the treatment.
The closing sleeve acts primarily as an sand exclusion device and should be closed immediately after screen out to minimize the potential for proppant flow back. To contend with erosional effects two closing sleeve designs are utilized. For extreme rates or amounts of proppant, a long ported sleeve is recommended. The ported sleeve design protects the sealing surface by blocking erosion with a non-critical area of the tool. A less costly short sleeve relies on precisely directing the erosion effects away from the critical sealing surface.
To have the possibility to reverse out (Option II, as set forth below), two concentric strings of wash pipe need to be run between the top packer and the service tool; it can be either two independent strings or joints of pipe that have been made with adapter subs on top and bottom and with the annulus already assembled. An additional port will have to be added above the service tool, with a cup facing down on each side of the port; to reverse, fluid will be pumped into the annulus, out the port, around the bottom facing cup, into the pumping ports and up the tubing string; depending on the reservoir's resistance to treatment fluid injection, this will also require the use of a downhole pack-off, commonly referred to as a stripper below the top packer. Reversing out may commence once the treatment is completed by pumping down the casing/work string annulus, into the concentric service tool string annulus and around the inner isolating cup.
With the mechanical reversing/circulating valve, reversing out is enhanced by opening a flow path from the concentric tool string annulus into the lower area of the straddle or cup containment area. The flow path is opened and the lower wash pipe is closed off by raising the service tool string as little as 3 inches.
With the dump valve, in case the rat hole is full of treatment sand, the option is to pick-up the service tool into the blank pipe below the top packer into an area where only the bottom cups of the service tool effect a seal; the operator can then develop a reverse flow condition to remove the excess sand and can then run the service tool back down to its servicing position. A fixed screen below the sump packer is used to ensure separation of slurry while dumping in the lowest zone.